Luminaires having gas discharge lamps, ballasts for gas discharge lamps and methods for operating same are comprehensively known. A gas discharge lamp, occasionally also called gas discharge tube, discharge tube or the like, usually serves to provide light on the basis of electrical energy supplied. For this purpose, the gas discharge lamp includes at least the two electrodes arranged in a manner spaced apart with the predefined spacing in the transparent discharge vessel. The discharge vessel is generally hermetically sealed and filled with the gas. An electrical voltage is applied to the electrodes, such that an electric field forms between the electrodes.
For this purpose, an electrical potential can be applied to the electrodes from outside via electrical lines. The electrodes are usually connected to the ballast that applies the electrical voltage to the electrodes, such that the desired gas discharge for the purpose of generating light can be effected.
The discharge vessel is often formed from a ceramic material such as glass, in particular quartz glass, aluminum oxide ceramic or the like.
In general, the gas in the discharge vessel has a low pressure at room temperature when not in operation as intended.
The gas can be formed from a single gaseous substance or else by a gas mixture including a plurality of different gaseous substances. Furthermore, provision can also be made, of course, for the gas to evolve to its desired composition at a later point in time during operation on account of evaporation of a solid and/or a liquid within the discharge vessel. The gas need not have a constant composition. The gas composition can be dependent on a respective operating situation of the gas discharge lamp.
Certain properties of the discharge can be determined by means of the composition of the gas. Heat is liberated, inter alia, as a result of the gas discharge, said heat having the effect that a pressure in the discharge vessel increases. As gas it is possible to employ substances or substance mixtures such as, for example, metal vapors of sodium, rare earth metals, mercury and/or the like and also, if appropriate, with addition of halogens, for example in the manner of a metal-halide lamp or the like. Furthermore, also for promoting the ignition of a gas discharge lamp, a noble gas can be provided in the gas, for example xenon, krypton, neon or the like and also mixtures of halogens and further metals.
In order to generate high luminances, gas discharge lamps can be operated in the manner of high-pressure gas discharge lamps or else extra-high-pressure gas discharge lamps. Here the discharge is effected in the region of an arc that forms between the electrodes, such that an arc discharge is provided. High-pressure gas discharge lamps are available for example as mercury-vapor lamps, krypton arc lamps or the like. During operation as intended, a pressure of up to approximately 1 MPa can be present in the case of such gas discharge lamps. In the case of extra-high-pressure gas discharge lamps, such as, for example, the extra-high-pressure mercury-vapor lamp, the xenon short-arc lamp or the like, the pressure in the gas during operation as intended can even be up to approximately 10 MPa or even more, for example approximately 20 MPa or even 30 MPa.
Particularly in the case of high-pressure gas discharge lamps and also in the case of extra-high-pressure gas discharge lamps, the electrodes are often formed from tungsten. The respective electrode can be designed for example in a pin- or rod-like fashion and, if appropriate, additionally also include a wire winding. In the case of these gas discharge lamps, a current density in the gas is generally so high that a low-pressure discharge upon starting immediately transitions to an arc discharge, such that the internal pressure increases greatly as a result of increasing temperature and possible evaporating filling constituents. Besides the electrodes that form the operating electrodes, in addition ignition electrodes can also be provided, for example in the case of mercury-vapor lamps or the like.
A generic ballast and also a generic method for operating same are described in EP 1 594 349 A1. The ballast includes a clocked energy converter in the manner of a step-down converter or a step-up converter, which serves to supply a lamp with electrical energy from an energy supply system. The ballast furthermore includes a damping element for reducing radio interference.
Furthermore, EP 0 837 620 A2 discloses a ballast and a method for operation for a metal-halide lamp and is concerned with the problem that convection within the discharge vessel of the metal-halide lamp during operation as intended causes an undesired curvature of the discharge arc if the electrodes of the metal-halide lamp, which are arranged with a spacing of approximately 3 mm, during operation as intended are not arranged in the upright position but rather in a horizontal position, for example, that is to say that a gap between the electrodes is horizontally aligned. In order to combat this problem, a high-frequency ripple current is superposed on a low-frequency rectangular-waveform current by the electrodes. This results in straightening of the arc. Nevertheless, undesired oscillations in a periphery of the arc discharge arise as a result if an amplitude of the ripple current exceeds a certain threshold value. In order to counteract this problem, EP 0 837 620 A2 proposes amplitude modulation of the high-frequency ripple current with a low-frequency signal.
Even if the abovementioned teaching has proved worthwhile, further disadvantages are nevertheless manifested. Firstly, flicker in the region surrounding the arc, also called halo, can be suppressed only to a limited extent and, secondly, a considerable control complexity is required in the ballast in order to implement the teaching of EP 0 837 620 A2. This is very complex and expensive and furthermore requires a considerable structural space.